The present invention relates to a method of producing a weld overlay on a ferrous base metal surface, such as a low alloy steel or cast iron, by electric arc welding, and to a product produced by the method such as a steel mill roll, having improved machinability, metal-to-metal wear resistance and corrosion resistance. Although not so limited, the invention has particular utility in the production of or rebuilding of rolls for continuous casting machines, hot strip mill run-out table rolls, boat shafts and equipment for handling mineral materials and coal.
Conventional rolls for uses of the type set forth above comprise a core section of low alloy steel or cast iron having an overlay of ferritic or martensitic stainless steel. AISI Type 420 martensitic stainless steel is widely used for this purpose. The nominal composition of Type 420 is over 0.15% carbon, 1.00% maximum manganese, 1.00% maximum silicon, 0.040% maximum phosphorus, 0.030% maximum sulfur, 12.00% to 14.00% chromium, and balance essentially iron. Martensitic weld overlays include alloys which may be ferritic but are transformed to a substantially fully martensitic structure by appropriate austenitizing heat treatment and cooling.
U.S. Pat. No. 4,404,450 discloses a roll welding machine and a method of reconditioning caster rolls, wherein a Type 420 martensitic stainless steel is applied as an overlay by submerged arc welding. The processing disclosed in this patent includes the conventional steps of preheating the roll to a minimum preheat temperature of 600.degree. F., with the preheating being conducted at a rate of 150.degree. F. per hour. The roll is maintained at the desired temperature of at least 600.degree. F. throughout the welding operation.
U.S. Pat. No. 4,436,791 discloses the production of a cast steel roll comprising a core or heart of nodular or lamellar cast iron and an outer case of martensitic steel having a chromium content of 8% to 16% and a carbon content of 0.65% to 0.95%. The residual austenite content of the case metal is less than 10%, and the hardness of the case exceeds 700 HV. The roll of this patent is produced by casting, followed by an austenitization treatment at a temperature above 900.degree. C. for 8 to 24 hours, cooling in air to a temperature of 400.degree.-500.degree. C., maintaining the temperature for 8 to 24 hours, air cooling to room temperature, and tempering for 8 to 24 hours at a temperature of 400.degree. to 450.degree. C. The case metal is balanced with respect to the carbon and chromium contents so as to reach a slightly hypereutectoid composition which avoids the presence of harmful ledeburitic carbides.
U.S. Pat. No. 4,420,335 discloses a steel for roll materials in continuous casting equipment, comprising 0.04% to 0.20% carbon, 0.2% to 0.8% silicon, 0.4% to 1.5% manganese, 0.2% to 1.0% nickel, 10.0 to 14.0% chromium, 0.5% to 4% copper, 0.1% to 0.5% vanadium, 0.01% to 0.35% niobium (columbium), 0.01% to 0.06% aluminum, phosphorus and sulfur up to 0.03% each, and balance iron. In one exemplary embodiment a weld overlay was applied to a roll core. Good weldability is alleged for the steel compositions disclosed in this patent, particularly after heat treatment for the purpose of hardening by transformation to martensite, and tempering.
Japanese published application 57169-067 discloses a roll having a surface layer of a ferritic steel containing up to 0.1% carbon, 10.0% to 14.0% chromium, 0.4% to 1.0% columbium, and balance iron with usual impurities. Preferably the surface layer is formed by welding, and the layer may contain a small amount of carbide precipitates formed by the columbium addition.
Japanese published application 58193-343-A discloses a composite roll having an outer layer of high chromium graphite cast iron comprising 2.4% to 3.4% carbon, 2.0% to 3.4% silicon, 0.5% to 1.5% manganese, up to 0.08% phosphorus, 4.5% to 10% nickel, 5% to 10% chromium, 0.4% to 1.5% molybdenum, and balance iron. Optionally up to 1.0% total of one or more of columbium and vanadium may be present. The outer layer is applied by welding to a core of cast iron, ductile cast iron or graphite cast steel.
Apparatus for application of weld overlays on shafts is disclosed in U.S. Pat. Nos. 4,322,596 and 4,295,592.
It is clear from the above prior art that the production of new or rebuilt steel mill roll surfaces requires lengthy and expensive preheat treatment, welding under carefully controlled temperature conditions and stress relief heat treatment subsequent to welding. Where a martensitic case or outer layer is used, still further heating and cooling steps of substantial duration are required for transformation of the outer layer to martensite.
A temporary product data bulletin (TPD-3) was published in April, 1983 by the assignee of the present application disclosing the composition, mechanical properties, wear resistance and corrosion resistance of the alloy in other product forms. It suggests possible use as a weld overlay in surface wear applications such as steel mill rolls.
Despite the processing precautions and composition limitations disclosed in the above prior art publications, steel mill rolls presently available frequently exhibit inadequate metal-to-metal wear resistance, crack propagation resistance and corrosion resistance.